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What Kind of Backup Generator Do YOU Need?

by Michael Chotiner


There's an irony in the fact that the times when we need electricity the most—times of emergency—are also the times when it's the hardest to come by. After a deep freeze or a big storm, we want to get news of the damage, contact loved ones (on phones that can't be charged!), and most importantly, stay warm and safe.

For many homeowners, a generator is an excellent tool to have on hand. But with so many options out there, you're bound to have a few questions: what type, how big and how much?

Portable generator graphic courtesy Home DepotStandby vs Portable Generators... a lifestyle choice!

When considering generator types, first ask yourself: how much work you want to do to set up, maintain and start up the generator when the power goes out?

Standby Generator... easy to use and more expensive to own but better payback in the long run

For the greatest convenience, a permanently-installed backup generator is the best bet. Backup generators are wired into a building's electrical system via a transfer switch. They sense when power from a utility provider is interrupted and start up automatically. The most convenient models are self-monitoring and start themselves up at regular intervals to maintain readiness for emergencies. Available models can run on natural gas, liquid propane, gasoline or diesel fuel.

Remodeling Magazine's 2014 Cost vs. Value Survey reports that the average installed cost of a built-in standby generator is $11,742. Some households can do it for less, especially if they don't get a unit that supplies more power than they actually need to get through a temporary outage. But whatever the cost of the generator and transfer switch, you're also in for the cost of professional installation, permit and inspection fees. The survey also calculates that homeowners recover less than 70% of the investment in a standby generator on resale of their homes.

Portable Generators... for the frugal and the do-it-yourselfer who likes portability and flexibility

Much less expensive than a built-in generator, a portable can get you through a blackout. You'll spend between $300 and $2,500 for a portable, depending on output. But a portable generator isn't a turnkey backup system: in an emergency, you'll need to roll the generator outdoors, start it up and run extension cords to the lights and appliances you want to power. You should also start and test the generator a couple of times a year to be sure it'll be ready when you need it.

Portable generators run on gasoline or diesel fuel. Their engines spew carbon monoxide when running. They should be run outdoors only, at least 10 feet from the house. And your unequipped neighbors won't only be envious if you have lights and they don't—they're likely to object to a portable generator's noise.  (Unless they have one themselves!)

Determine how much power you need...

Whether you're considering a permanent standby model or a portable generator, you've got to calculate how many watts of electricity it will have to provide to keep whichever appliances you consider essential running simultaneously during a power failure. Since the cost of a generator is directly related to its power output, it's most economical to consider only those appliances you really can't do without for a few days.

The table to the left lists the appliances and wattage requirements of appliances that might be considered essential. To use it to calculate your own needs, check off the ones you think you'll really need, and go around your house to find the wattages of appliances important to you that I may have omitted. You can usually find the wattage rating on the label that also gives the model number and year of manufacture.

Typical power requirements of household appliances
Values derived from:

Once you've identified your "must-haves", add up all the wattages and multiply the sum by 1.5. (Appliances with motors, like refrigerators and washing machines, need more power to start up than to run). That'll give you the minimum output for the generator you'll want.

Sizing a Permanent Standby Generator

If you're considering a whole-house generator, there's an easier way to determine the output you'll need based on the electric service in your home. See the table below, which correlates generator outputs with various standard capacity service panels. A generator that delivers more than 20% of the normal service can get you through a power outage, if you're selective about running critical appliances; one that provides about half the wattage of a given service panel should enable you to live large without thinking too much about it.

Standy generator power computation
Values derived from:

Trade-Offs between Standby and Portable Generators...

To run your whole house without a hiccup, you could go for a 20,000-watt built-in option, which would supply about 40% of the power of a typical 200-amp service—probably enough for normal operation. I've found suitable models online that would cost about $6,500 installed.

If that's out of your budget, a portable model can still suffice. To run the cable box and TV, clothes washer, gas dryer, electric range, boiler pump, a few computers and lights—all at the same time—you'd be safe with an 8,000-watt generator. A cart-mounted portable model in that range will likely cost between $1,200 and $1,500.

Portable Generators and Extension Cords

By itself, a portable generator can supply power to a given appliance only when connected to it with an extension cord. Most portables have a number of receptacles with different configurations, including standard 3-prong, 120-volt outlets and 240-volt outlets for high-demand appliances like electric ranges. Extension cords used with a portable generator should be the outdoor type and at least 14-gauge.

Apart from the hassle, connecting appliances to a generator with extension cords presents other issues, like the need to leave a door or window open to route them inside. Another problem is that certain appliances you may need to run during an outage—like a well or sump pump, a furnace blower or a central air conditioner—can't be plugged in with extension cords.

Transfer Switches Make Portable Generators Safer and Easier To Use...

It's much easier and safer to run power from a portable generator to your home's electric service panel through a transfer switch—$300 to $500, plus installation by a licensed pro. One cord from the generator to the switch is all you'll need. Suitable transfer switches enable power produced by the generator to be routed to six to ten circuits—those that serve your critical appliances.

A transfer switch is also a safety must for any generator connected to house wiring. It keeps generator power from being sent out over downed power lines, which could be very dangerous for repair workers!

I'll be ready for the next blackout in my neighborhood. Are you?

About the author:  DIY expert Michael Chotiner provides valuable advice and tips for fellow homeowners for Home Depot. Michael is a carpenter who has run his own construction business. A large variety of home generators available at Home Depot, including types referenced by Michael, can be found on the company's website.